Apparatus for desalting electrolyte aqueous solutions is known and in particular there are electrodialyzers which comprise a number of chambers formed in the housing of the apparatus by partitions of selective ion-exchange membranes through which solutions of salts are moved. In the end chambers there are electrodes connected up to the direct electric current supply. Applied across the membranes, the electric current makes the chambers in turn enriched and poorer with electrolyte. This process results in separation of the initial solution of electrolyte into desalted and concentrated solutions.
As described in U.S. Pat. No. 4,525,259, an electrodialyzer comprises a line of concentrating chambers between which are situated chambers with diluted solution. This line of chambers has a cathodic chamber with a cathode at one end of it and an anodic chamber with an anode at another end.
In the chambers adjacent to the chambers which contain electrodes, the diluted solution is maintained to prevent migration of ions from concentrated solutions in the concentration chambers into the chambers with the electrodes in them.
All the chambers of this electrodialyzer have input and output pipe branches for corresponding aqueous solutions. Also, the apparatus has two tanks for catholyte and anolyte collection which are in communication with corresponding electrode chambers. These tanks may be isolated from the electrode chambers during the process of solution recirculation between the chambers.
Another common electrodialyzer (F. N. Meller, "Electrodialysis--Electrodialysis Reversal Technology", IONICS, Incorporated, March 1984, p. 53-56) comprises a housing inside of which are set alternately with cation- and anion-exchange membranes arranged parallel with each other to form a line of chambers; a anodic chamber with an anode set at one end wall and a cathodic chamber with a cathode at another end wall with at least one pair of concentration/desalting chambers between them. The anodic chamber is formed by the internal surface of the housing walls and by one side of the first cation-exchange membrane, the other side of which, together with the internal surface of the housing walls and one side of the anion-exchange membrane, forms a concentration chamber which is adjacent to the anodic chamber. Another side of the anion-exchange membrane together with the internal surface of the housing walls and the next cation-exchange membrane form a desalting chamber.
The rest of the pairs of concentration/desalting chambers are formed in the same way. The last cation-exchange membrane, one side of which limited the adjacent desalting chamber, forms the cathodic chamber by its other side and the internal surface of the housing wall.
Each chamber of this known electrodialyzer has an input pipe branch for initial aqueous solution. What is more, anodic and cathodic chambers have input pipe branches for anolyte and catholyte respectively, while each concentration and desalting chamber has output pipe branches for concentrated and desalted solutions.
This known electrodialyzer is equipped with a relay unit for electrode polarity reversal and with a device for redirecting hydraulic flows from the desalting and concentration chambers.
The following describes the normal operation of the known electrodialyzer.
An aqueous solution of electrolyte is fed simultaneously into all chambers through the respective pipe branches, then a direct electric current is supplied to the electrodes.
The process of electrolysis takes place in anodic and cathodic chambers and results in anolyte and catholyte formation respectively. Under the influence of the direct current the ions of the salts diffuse through ion-exchange membranes into the concentration chamber realizing the desalting of aqueous solution in the desalting chamber and concentrate formation in the concentration chamber. Then the desalted solution and the concentrate are withdrawn out of the chambers via the respective pipe branches.
Electrolysis leads to a disturbance of the conditions of electric neutrality of aqueous solution at the interface between the ion-exchange membrane and the aqueous solution, and inevitably results in the formation of hardly soluble salts on the ion-exchange membranes. These salts make permeability of membranes difficult. Owing to loss of conductivity the process of desalting stops. To restore the desalting ability of the electrodialyzer it is then necessary to reverse the electrode polarity and reverse the directions of hydraulic flows from the desalting and concentration chambers and thus change the direction of ion migration through the ion-exchange membranes. Electrochemical dissolution of sedimentary salts results in formation of an intermediate solution which is brought out of both chambers of each pair.
Then the first output pipe branches are set shut while the second output pipe branches in the desalting and concentration chambers are set open such that the functions of these chambers interchange their places and the concentrate and the desalted solution are brought out of chambers via the second pipe branches respectively.
When the desalting ability of the electrodialyzer is restored, the working cycle is repeated.
A common disadvantageous feature of prior art devices such as that of the electrodialyzer which has been described herein, consists of low specific productivity caused by the inevitable formation of the sedimentary hardly soluble salts on the ion-exchange membranes and the resulting loss of conductivity necessitating a halt of the process of aqueous electrolyte solution desalting. The need to periodically restore desalting apparatus, caused by the formation of the sedimentary hardly soluble salts on the membranes, requires additional means and measures to eliminate them.
Thus, the common design of the electrodialyzer is complicated due to the units for electrode polarity reversion and for turning hydraulic flow in directions opposite to their working ones. Besides, additional energy expenditures for electrochemical dissolution of these sedimentary salts are necessary.
What is more, due to the need to feed the initial solution into each chamber and the need to form and withdraw intermediate- and end-products from each chamber, the usual design of the electrodialyzer is also complicated by the large number of pipe branches which result in inconvenience in assembly and maintenance and also leads to an increase in the total material required.
Another known electrodialyzer is described in UK Patent 882 432 comprising a succession of interlacing desalting and concentration chambers with cationic and an ionic exchange membranes successively arranged and with an anodic chamber and a cathodic chamber at opposite ends of the electrodialyzer. This apparatus can run a two or three flow process creating conditions for controlling cation transference to the anodic chamber due to replacement of a cation-exchange membrane with an ion exchange membrane.
However, this known apparatus has the disadvantage of attracting deposition of hardly soluble calcium sulphate, calcium carbonate, magnesium and iron hydroxides on the membranes in the desalting chamber and this leads to a decrease in productivity and additional energy consumption.
In UK Patent 750 500 desalination takes place in compartments formed by cation and anion exchange membranes with demineralized water flowing in one direction and concentrate flowing in the opposite direction. The initial water flow into the anodic chamber is controlled. It flows from the anodic chamber to the concentration chamber from which the concentrate is removed from the electrodialyzer.
Again this known apparatus attracts undesirable deposition of hardly soluble salts on the membranes and on the side of the desalting chambers. Not only does this reduce its effectiveness and increase its energy consumption but it is also necessary to regularly halt the process to clean off the deposits thus further reducing productivity.
In WO91/04782 a succession of chambers is described including an anodic chamber into which the initial water solution is fed and in which acid synthesis occurs on the inner side of the anode. The acid produced is fed to a desalting chamber in a hermetically sealed channel and then diffuses through cation-exchange membranes into concentration chambers. A pH of 3 and less is thus possible.
In this known electrodialyzer the acid synthesis proceeds due to water oxidation according to the formula EQU 2H.sub.2 O-4e.fwdarw..DELTA.H.sup.+ +O.sub.2 I
Thus it can be seen that during acid synthesis, bubbles of gaseous hydrogen form on the anode and cover its surface thus reducing the active surface of the anode and increasing the energy consumption.
These bubbles form gas sacks in the desalting chambers and water flow channels causing increased hydraulic resistance, and reduction of the ionic transference rate through the membranes. Again lower productivity and higher energy consumption results.
Another problem with this known apparatus is the phenomena of concentration polarization at the anode solution interface due to a lower concentration gradient in the diffusing layer because of the low anion rate of diffusion from the solution to the anode reaction surface. This causes undesirably high water decomposition when the flow density is particularly high and again additional energy is consumed and lower productivity results.
In FR-A-1 223 965 an electrodialyzer is described with a succession of chambers, including a concentration chamber, separated by anion and cation-exchange membranes. However problems are also encountered in this apparatus in that deposition of salts on the membranes reduces productivity and increases energy consumption.
It will therefore be seen that known electrodialyzers are unsatisfactory and improvements are desirable in particular because of the deposits of hardly soluble salts on the membranes resulting in increased energy consumption, and also because removal of sediment through acid synthesis at the anode, necessitating a pH of 3 or less, and the further transference to the concentration and desalting chambers requires an extremely high current density on the electrodes resulting in undesirably high energy additional consumption.
The present invention seeks to provide an improved electrodialyzer compared to the known apparatus.